The company, which remains the world's number two mobile phone maker behind Samsung but is outside of the top five in a smartphone market dominated by the likes of Apple and Samsung, has described the deal as "the best path forward".
The company also announced the immediate departure of chief executive Stephen Elop, to be replaced in the interim by Risto Siilasmaa, Nokia's chairman.
Of the total purchase price of 5.44 billion euros, 3.79 billion relates to the purchase of Nokia's Devices & Services business, and 1.65 billion relates to the mutual patent agreement and future option.
Microsoft investing in its mobile future
Microsoft has struggled in recent years as consumers have transitioned from desktop and laptop PCs to mobile devices.
While its Windows software is used on the vast majority of personal computers, Microsoft has had little impact in the fast-growing segments of tablets and smartphones.
"Microsoft has clearly been on the wrong track for a long time," said Roger Kay, analyst at Endpoint Technologies Associates. "Essentially Microsoft missed the shift to high mobility entirely."
Microsoft chief executive Steve Ballmer told reporters in a conference call that Windows Phone was "the fastest growing smart platform today, growing by 78 per cent last year".
"Today's agreement will accelerate our success in smartphones," he added.
Forrester analyst Clement Teo was likewise upbeat, saying that the "long-awaited" deal between Nokia and Microsoft will lead "more integrated apps and services".
"This acquisition is a clear stepping stone in Microsoft's transition from a software company to a software-led multiproduct company," he said.
Ben Kepes, founder and principal analyst of New Zealand analysis firm Diversity, described the deal as a "marriage of convenience", telling the International Business Times that: "Microsoft gets to follow in